Murray Gell-Mann died on May 24, 2019, in his Santa Fe home after a long and debilitating illness. He was among the greatest theoretical physicists of the twentieth century, as well as my mentor, champion, and friend for 60 years… even though he never understood my reluctance to accept the superstring as a promising approach to particle physics.
I first encountered Gell-Mann in the spring of 1959, when he invited me to describe my work at a seminar in Paris. Having completed my Harvard thesis with Julian Schwinger, I was spending my first postdoctoral year in Copenhagen at what would become known as the Niels Bohr Institute. Gell-Mann was on sabbatical at the Collège de France. Not yet 30, he was already a renowned theorist. Among much else, he had explained the puzzling features of what were called strange particles. Gell-Mann proposed a new particle attribute S, which he called strangeness. He assumed it to be conserved by the strong nuclear force but not by the weak.1 Ordinary particles, like protons and neutrons, have no strangeness, whereas strange particles are variously assigned S = ±1 or ±2. They can be produced copiously by energetic particle collisions, but always two at a time and never singly. Their lifetimes are so unexpectedly long because their decays necessarily involve the weak force. Puzzle solved!
After my seminar, Gell-Mann invited me to a tête-à-tête dinner at a two-star Michelin restaurant, where I was cured of my lifelong aversion to fish as food. Gell-Mann seemed to appreciate my algebraic explanation for the universality of weak and electromagnetic coupling strengths. He presented my ideas, duly crediting me, at the 1959 International Conference on Elementary Particle Physics in Kiev, which I, being a mere postdoc, could not attend.
A year later, as my two-year National Science Foundation fellowship was running out, I was surprised and delighted to receive an invitation from Gell-Mann to spend a third postdoctoral year at Caltech. I accepted immediately: to learn from such luminaries as Richard Feynman and Gell-Mann, to enjoy California’s warm weather, and because I had few alternatives. Soon after arriving in Pasadena, I encountered Gell-Mann’s graduate student Sidney Coleman, who would become my lifelong friend, collaborator, and colleague at Harvard. Conversations with Gell-Mann were always both rewarding and frustrating. As I began explaining a new idea I might have, he would instantly develop the idea and explain its consequences to me. He knew more than almost anyone about almost anything, including birds, flowers, rocks, foods, languages, mushrooms, and cacti, but not music, which he did not much care for.
Early in 1961, Feynman and Gell-Mann developed a sudden interest in Lie groups. They huddled together on a work that would be named “The Eightfold Way: A Theory of Strong Interaction Symmetry.”2 Feynman chose to withdraw from what he thought was a speculation too far. Sidney and I, contrariwise, were enchanted by Gell-Mann’s theory. We explored its consequences, deduced the successful Coleman–Glashow mass formula, a minor supplement to the Gell-Mann–Okubo mass formula, and became world-traveling disciples of the eightfold way, laying bets on its validity en route.
The year 1964 was extraordinary for fundamental physics, second only to Albert Einstein’s annus mirabilis of 1905. Early in the year, Gell-Mann proposed the existence of fractionally charged quarks as the constituents of nucleons and all other strongly interacting particles. He invoked three different kinds of quark, which he referred to as flavors: up and down quarks, and a heavier strange quark. The proton, for example, was made of two up quarks and a down quark. Strange particles are simply those containing one or more strange quarks or antiquarks, and Gell-Mann’s eightfold-way symmetry scheme emerges naturally from the quark model.3 In the summer of 1964, James Bjorken and I, for reasons of elegance and symmetry, posited the existence of a fourth charmed quark. Particles containing charmed quarks were observed a decade later. Today we know of two additional quark flavors, top and bottom, making a total of six, which happens to be the first perfect number!4
In September of the same year, Nicholas Samios at Brookhaven National Laboratory announced his group’s detection of the omega-minus baryon. Gell-Mann had predicted the existence and properties of this particle two years previously on the basis of his eightfold way. The newly discovered particle had just the mass, lifetime, and decay scheme that Gell-Mann had foreseen. The doubters were silenced. All bets placed by Sidney and me were paid.
While empirically it was very successful, the quark model had a big problem. Quantum theory requires quark wave functions to change sign when any two quarks are exchanged, but this condition seemed impossible to satisfy by the three identical strange quarks within the omega-minus particle. In November, Oscar Greenberg addressed this problem with what he called parastatistics of order three. Gell-Mann found a simpler solution by assigning yet another attribute to quarks: color. Quarks of each flavor were to come in three different colors. The required antisymmetry of the omega-minus wave function of each of its strange quarks is differently colored. A decade later, Gell-Mann and his collaborators incorporated quark color into quantum chromodynamics, or QCD, today’s widely accepted candidate for the strong force. The force holding quarks together to form nucleons results from the exchange of colored gluons. A residue of this force holds nucleons together to form atomic nuclei. Another virtue of quark color is to provide an arena in which QCD can operate, leaving the arena of quark flavor open for the operation of electroweak forces, the other component of today’s triumphant Standard Model of elementary particle physics.
Other seminal advances in basic physics made in 1964 include time-reversal violation by the weak force; the Higgs mechanism, by which symmetries can spontaneously be broken; and the cosmic microwave background. These three discoveries led to the awarding of six Nobel Prizes in physics. It had been quite a year!
Gell-Mann made a significant impact on the English language with coinages such as quark, gluon, strangeness, and chromodynamics, and by giving the words flavor and color new senses having nothing to do with taste or vision. The entry for quark in the Oxford English Dictionary (OED) includes Gell-Mann’s letter explaining its etymology and pronunciation. Indeed, the OED contains 31 separate citations for Gell-Mann. This might be compared to 20 for Feynman, 17 for Albert Einstein, and a mere eight for my beloved Harvard advisor Schwinger. Perhaps the number of citations in the OED is not a meaningful measure of merit for a scientist.
I returned to Harvard as a professor in 1966. As evidence for quarks and QCD accumulated, Gell-Mann turned his attention away from particle physics and toward his many other disparate interests. When we offered him a position, I, along with Coleman who was then also at Harvard, tried our very best to convince him to join us. We almost succeeded. I am told that I received my share of the 1979 Nobel Prize in physics as a result of active interventions by both Gell-Mann and Coleman.
As the Standard Theory flourished, Gell-Mann became convinced that quantum field theory was not powerful enough to answer our remaining questions. He was an early advocate of string theory and for a decade offered strong support to his protégé John Schwarz, one of the founding fathers of string theory. In 1984 Gell-Mann cofounded the Santa Fe Institute for the study of complex phenomena. A decade later he published The Quark and the Jaguar: Studies of the Simple and the Complex, an excellent introduction to complexity.5 He wrote 10 papers with James Hartle between 1989 and 2014 trying to reconcile quantum measurement and cosmology. I last saw Gell-Mann in 2016. He recognized me, but was no longer able to converse.
